"Daniel (Ricciardo) proved last year that he can drive on-par with Vettel. We do not have a driver problem," he said.

But what about Daniil Kvyat? Red Bull elected to replace the Ferrari-bound Vettel with its 20-year-old Russian junior, and he has struggled notably in 2015.

Horner said: "Daniil had the best result of his career just in Monaco, which is a clear sign that things are looking up."

Another theory is that Red Bull's decline could be linked with the departure to McLaren of aerodynamicist Peter Prodromou, while Adrian Newey has stepped back from the front line.

Horner replied: "We have more than 700 employees in Milton Keynes, and the organisation is strong enough that we can absorb any departure.

"Adrian is still heavily involved in the formula one programme," he added. "If before he was working from Monday to Friday, now he's spending the first two days of the week on other projects."

Yet another theory is that, after its run of Vettel-era success in 2010-2013, Red Bull is simply a little 'tired'.

"The opposite is true," Horner insists. "We are still the same team that won four drivers' and four constructors' titles, and we are hungry for more."

Whether Red Bull can achieve that again with Renault, however, is another story, as both sides appear particularly unhappy with the other in 2015.

But there is an existing contract through 2016, as Horner explains: "In the coming weeks we will work together to decide what the future looks like.

"Red Bull has invested a lot in formula one and we want to be here but we want to be competitive. Should Renault decide to leave the sport, we will be forced to act."

He denied, however, that Red Bull will make its own engine, and played down reports of talks with Audi.

"There are no negotiations with Audi, and the sale of the team is also not an issue," said Horner.

And he now sees some light at the end of the tunnel.

"In the first races (of 2015) we had to compromise the chassis in order to support Renault. But in Spain and Monaco we returned to a more normal level and that was reflected in the results," said Horner.

And he said he is expecting a performance improvement from Renault "between Austria and Silverstone".